Noise rejection for optomechanical devices

ABSTRACT

An optomechanical device comprising a circuit configured to generate an optical signal using a tuning signal and modulate the optical signal at a frequency corresponding to one quarter of a Full Width at Half Maximum (FWHM) of an optical resonance of the proof mass assembly to generate a partially modulated optical signal. The circuit being further configured to filter the partially modulated optical signal to remove a central carrier from the partially modulated optical signal to generate a filtered optical signal, modulate the filtered optical signal to generate a modulated optical signal driven to the mechanical resonance of the proof mass assembly, and generate the tuning signal using a difference between a DC intensity level of a first optical frequency component in the modulated optical signal and a DC intensity level of a second optical frequency component in the modulated optical signal.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under N66001-16-C-4018 awarded by SPAWAR Systems Center Pacific. The Government has certain rights in the invention. This material is based upon work supported by the Defense Advanced Research Projects Agency (DARPA) and Space and Naval Warfare Systems Center Pacific (SSC Pacific).

TECHNICAL FIELD

This disclosure relates to optomechanical devices, such as accelerometers configured to measure acceleration using a modulated optical signal.

BACKGROUND

Optomechanical devices include devices for detecting acceleration, velocity, vibration, and other parameters. For example, in an optomechanical accelerometer, the resonance frequency of a mechanical structure is shifted under acceleration in the optomechanical device. The mechanical resonance frequency can be read out with an optical field by applying near-resonant light to the structure's optical resonance and measuring the transmitted or reflected optical signal.

SUMMARY

In general, the disclosure is directed to devices, systems, and techniques for noise rejection for optomechanical devices. As used herein, noise rejection may refer to mitigating effects of fluctuations (e.g., small fluctuations) in amplitude or frequency that shift a mechanical resonance frequency (e.g., via the optical spring effect). Such fluctuations in amplitude or frequency may limit the ultimate noise floor and hence performance of the optomechanical device. Specifically, a circuit of an optomechanical device may be configured to modulate an optical signal into a first optical frequency component corresponding to the optical resonance plus one quarter of the Full Width at Half Maximum (FWHM) of the optical resonance, and a second optical frequency component corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance. In this example, the circuit may generate a tuning signal for driving a light-emitting module using a difference between a Direct Current (DC) intensity level of the first optical frequency component in the modulated optical signal and a DC intensity level of the second optical frequency component in the modulated optical signal. In this way, the circuit may spectrally separate the response of each optical frequency component to mitigate effects of fluctuations in amplitude and frequency, which may mitigate the effects of fluctuations in amplitude or frequency.

In one example, an optomechanical device for tracking a mechanical resonance of a proof mass assembly includes the proof mass assembly and a circuit. The circuit is configured to generate an optical signal comprising a central carrier corresponding to an optical resonance of the proof mass assembly using a tuning signal, modulate the optical signal at a frequency corresponding to one quarter of a FWHM of the optical resonance of the proof mass assembly to generate a partially modulated optical signal comprising the central carrier, a first optical frequency component corresponding to the optical resonance plus one quarter of the FWHM of the optical resonance, and a second optical frequency component corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance, filter the partially modulated optical signal to remove the central carrier from the partially modulated optical signal to generate a filtered optical signal, modulate the filtered optical signal to generate a modulated optical signal driven to the mechanical resonance of the proof mass assembly, and generate the tuning signal using a difference between a DC intensity level of the first optical frequency component in the modulated optical signal and a DC intensity level of the second optical frequency component in the modulated optical signal.

In another example, a method for tracking a mechanical resonance of a proof mass assembly includes generating, by a circuit, an optical signal comprising a central carrier corresponding to an optical resonance of the proof mass assembly using a tuning signal, modulating, by the circuit, the optical signal at a frequency corresponding to one quarter of a FWHM of the optical resonance of the proof mass assembly to generate a partially modulated optical signal comprising the central carrier, a first optical frequency component corresponding to the optical resonance plus one quarter of the FWHM of the optical resonance, and a second optical frequency component corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance, filtering, by the circuit, the partially modulated optical signal to remove the central carrier from the partially modulated optical signal to generate a filtered optical signal, modulating, by the circuit, the filtered optical signal to generate a modulated optical signal driven to the mechanical resonance of the proof mass assembly, and generating, by the circuit, the tuning signal using a difference between a DC intensity level of the first optical frequency component in the modulated optical signal and a DC intensity level of the second optical frequency component in the modulated optical signal.

In another example, an optomechanical system for tracking a mechanical resonance of a proof mass assembly, the optomechanical system includes a circuit configured to generate an optical signal comprising a central carrier corresponding to an optical resonance of the proof mass assembly using a tuning signal, modulate the optical signal at a frequency corresponding to one quarter of a FWHM of the optical resonance of the proof mass assembly to generate a partially modulated optical signal comprising the central carrier, a first optical frequency component corresponding to the optical resonance plus one quarter of the FWHM of the optical resonance, and a second optical frequency component corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance, filter the partially modulated optical signal to remove the central carrier from the partially modulated optical signal to generate a filtered optical signal, modulate the filtered optical signal to generate a modulated optical signal driven to the mechanical resonance of the proof mass assembly, and generate the tuning signal using a difference between a DC intensity level of the first optical frequency component in the modulated optical signal and a DC intensity level of the second optical frequency component in the modulated optical signal.

The summary is intended to provide an overview of the subject matter described in this disclosure. It is not intended to provide an exclusive or exhaustive explanation of the systems, device, and methods described in detail within the accompanying drawings and description below. Further details of one or more examples of this disclosure are set forth in the accompanying drawings and in the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an electro-opto-mechanical system, in accordance with one or more techniques of this disclosure.

FIG. 2 is a block diagram illustrating the circuit of FIG. 1 in further detail, in accordance with one or more techniques of this disclosure.

FIG. 3 illustrates a conceptual diagram of the proof mass assembly of FIG. 1 including a proof mass suspended within a frame by a first doubled ended tuning fork (DETF) structure, a second DETF structure, and a set of tethers, in accordance with one or more techniques of this disclosure.

FIG. 4 illustrates a conceptual diagram of the electro-opto-mechanical system of FIG. 1, in accordance with one or more techniques of this disclosure.

FIG. 5 depicts additional aspects of the electro-opto-mechanical system of FIG. 1, in accordance with one or more techniques of this disclosure.

FIG. 6 is a conceptual diagram illustrating example techniques for noise rejection for an optomechanical device, in accordance with one or more techniques of this disclosure.

FIG. 7 is a conceptual diagram illustrating an example optical signal, in accordance with one or more techniques of this disclosure.

FIG. 8 is a conceptual diagram illustrating an example partially modulated optical signal, in accordance with one or more techniques of this disclosure.

FIG. 9 is a conceptual diagram illustrating an example filtered optical signal, in accordance with one or more techniques of this disclosure.

FIG. 10 is a conceptual diagram illustrating an example modulated optical signal, in accordance with one or more techniques of this disclosure.

FIG. 11 is a conceptual diagram illustrating an example optical response of a resonance in reflection as a function of wavelength of the optical signal, in accordance with one or more techniques of this disclosure.

FIG. 12 is a conceptual diagram illustrating an example optical response of a first optical frequency component and a second optical frequency component, in accordance with one or more techniques of this disclosure.

FIG. 13 is a flow diagram illustrating an example process for tracking a mechanical resonance of a proof mass assembly, in accordance with one or more techniques of this disclosure.

Like reference characters denote like elements throughout the description and figures.

DETAILED DESCRIPTION

This disclosure describes devices, systems, and techniques for noise rejection to mitigate the effects of fluctuations in amplitude and/or frequency. For example, in an optomechanical accelerometer, the resonance frequency of a mechanical structure can be shifted under acceleration in the optomechanical device. The mechanical resonance frequency can be read out with an optical field by applying near-resonant light to the structure's optical resonance and measuring the transmitted or reflected light. However, the application of the optical light field can also shift the mechanical resonance frequency (via the optical spring effect) in a manner that is independent of acceleration. Via this mechanism, small fluctuations in the amplitude or frequency of the applied light field can write mechanical frequency noise onto the device, which in turn, may be read out as acceleration noise on the optomechanical device.

Amplitude noise can come from several sources, including laser power fluctuations, optical coupling variability and photo receiver (e.g. a photodiode) calibration/aging effects. Frequency fluctuations can arise via laser frequency fluctuations or shifts of the optical resonance of the optomechanical device. This disclosure describes a circuit configured to reduce the effects of laser amplitude noise while simultaneously providing a process for frequency stabilization to the optical resonance. For example, by generating an optical signal comprising a first optical frequency component and a second optical frequency component with equal intensity but with equal but opposite detuning relative to the optical resonance, the amplitude dependence of the optical field can be nearly canceled. By spectrally separating the responses of each optical frequency, the optical signal can be stabilized to the optical resonance in a manner that greatly enhances the performance of the optomechanical device. While the foregoing examples were discussed with respect to acceleration, the optomechanical device may be configured to determine velocity, vibration, or another parameter.

For example, the optomechanical device may include an electro-opto-mechanical system configured to precisely measure very high acceleration values (e.g., up to 500,000 meters per second squared (m/s²)). The electro-opto-mechanical system may use a combination of electrical signals, optical signals, and mechanical signals to determine the acceleration of the object.

An optomechanical device may be configured to measure the acceleration, velocity, vibration, etc. of the object in real-time or near real-time, such that processing circuitry may analyze the acceleration, velocity, vibration, etc. of the object over a period of time to determine a positional displacement of the object during the period of time. For example, the optomechanical device may be a part of an Inertial Navigation System (INS) for tracking a position of an object based, at least in part, on an acceleration of the object. Additionally, the optomechanical device may be located on or within the object such that the optomechanical device accelerates, moves, vibrates, etc. with the object. As such, when the object accelerates, moves, vibrates, etc., the optomechanical device (including the proof mass) accelerates, moves, vibrates, etc. with the object. In some examples, because acceleration over time is a derivative of velocity over time, and velocity over time is a derivative of position over time, processing circuitry may, in some cases, be configured to determine the position displacement of the object by performing a double integral of the acceleration of the object over the period of time. Determining a position of an object using the accelerometer system located on the object—and not using on a navigation system separate from the object (e.g., a Global Positioning System (GPS))—may be referred to as “dead reckoning.”

The optomechanical device may be configured to achieve high levels of sensitivity in order to improve the accuracy of the acceleration, velocity, vibration, etc. values. High sensitivity may enable the optomechanical device to detect very small acceleration, velocity, vibration, etc. values, detect a very small change in acceleration, velocity, vibration, etc. values, detect a large range of acceleration, velocity, vibration, etc. values, or any combination thereof. Additionally, an optomechanical device may be configured to accurately determine the acceleration, velocity, vibration, etc. of the object while the object is experiencing high levels of acceleration, velocity, vibration, etc. In this way, the an optomechanical device may be configured to enable an INS to accurately track the position of the object while a magnitude of the acceleration, velocity, vibration, etc. of the object is very high.

The optomechanical device may, in some examples, include a micro-electro-mechanical system (MEMS) accelerometer which includes a light-emitting module, a circuit, and a proof mass assembly which includes a proof mass suspended within a frame by double-ended tuning fork (DETF) structures. In some examples, the optomechanical device may include a single-ended tuning fork or another structure.

In some examples, the DETF structures may be configured to carry optical signals. Additionally, optical signals may induce mechanical vibration in the DETF structures. In some cases, acceleration causes a displacement of the proof mass relative to the frame, the displacement affecting mechanical vibration frequencies (e.g., mechanical resonance frequencies) corresponding to the DETF structures. In this way, a mathematical relationship may exist between acceleration and the mechanical vibration frequencies of the DETF structures. As such, the mathematical relationship may be leveraged to determine acceleration. The accelerometer device uses, in some examples, a combination of optical signals and electrical signals to measure the mechanical vibration frequencies corresponding to the DETF structures and calculate acceleration based on the mechanical vibration frequencies.

While examples of an optomechanical device are described with respect to an example accelerometer, techniques described herein for noise rejection may be applied to optomechanical device configured to measure various parameters, including, but not limited to, acceleration, velocity, vibration, and other parameters. Moreover, while examples of the optomechanical device are described with respect to an example proof mass assembly that includes a DETF structure, other structures may be used, for example, but not limited to, a single-ended tuning fork structure or another structure.

FIG. 1 is a block diagram illustrating an electro-opto-mechanical system 10, in accordance with one or more techniques of this disclosure. FIG. 1 is merely one non-limiting example system architecture that may utilize the techniques of this disclosure for noise rejection. As illustrated in FIG. 1, system 10 includes light-emitting module modules 12A, 12B (collectively, “light-emitting module modules 12”), circuit 14, and proof mass assembly 16. Additionally, in the example illustrated in FIG. 1, circuit 14 includes electro-optic-modulators (EOM) 22A, 22B (collectively, “EOMs 22”), photoreceivers 24A, 24B (collectively, “photoreceivers 24”), feedback units 26A, 26B (collectively, “feedback units 26”), frequency counters 28A, 28B (collectively, “frequency counters 28”), and processing circuitry 30. While the example of FIG. 1 includes two EOMs, two photoreceivers, and two frequency counters, in some examples, an electro-opto-mechanical system may include only one EOM, one photoreceiver, and one frequency counter or more than two EOMs, two photoreceivers, and two frequency counters.

In the example of FIG. 1, light-emitting module 12A, proof mass assembly 16, EOM 22A, photoreceiver 24A, feedback unit 26A, and frequency counter 28A form a first positive feedback loop. Additionally, in the example of FIG. 1, light-emitting module 12B, proof mass assembly 16, EOM 22B, photoreceiver 24B, feedback unit 26B, and frequency counter 28B form a second positive feedback loop. In some examples, the second positive feedback loop may be omitted.

System 10 may be configured to determine an acceleration associated with an object (not illustrated in FIG. 1) based on a measured vibration frequency of a tuning fork structure of proof mass assembly. For example, system 10 may be configured to determine an acceleration associated with an object (not illustrated in FIG. 1) based on a measured vibration frequency of a set of double-ended tuning fork (DETF) structures which suspend a proof mass of proof mass assembly 16, where the vibration of the DETF structures is induced by an optical signal emitted by light-emitting module 12. In some examples, the first positive feedback loop generates a first frequency value representing a vibration frequency of a first DETF structure and the second positive feedback loop generates a second frequency value representing a vibration frequency of a second DETF structure. Based on the first vibration frequency and the second vibration frequency, system 10 may determine a first acceleration value and a second acceleration value, respectively. In some examples, system 10 determines an acceleration of an object based on the first acceleration value and the second acceleration value. In some examples, system 10 determines the acceleration of the object based on only the first acceleration value (e.g., the second positive feedback loop is omitted). In some examples, system 10 determines the acceleration of the object based on only the second acceleration value (e.g., the first positive feedback loop is omitted).

Light-emitting modules 12 may each include a laser device, also referred to herein as simply “laser,” configured to emit photons that form an optical signal. In some examples, light-emitting modules 12 emit the photons at an optical power within a range between 0.1 microwatts (μW) and 100 μW. In some examples, light-emitting modules 12 each include a semiconductor laser which includes a laser diode.

In some examples, circuit 14 may include a set of electrical components for processing and analyzing electrical signals received by photoreceivers 24. Components of circuit 14 are described in further detail below.

EOMs 22 may represent optical devices configured to modulate, based on electrical signals produced and processed by circuit 14, an optical signal emitted by light-emitting module 12. EOM 22A, for example, may include a set of crystals (e.g., Lithium Niobate crystals), where a refractive index of the set of crystals changes as a function of an electric field proximate to the set of crystals. The refractive index of the crystals may determine a manner in which EOM 22A modulates the optical signal. For example, the crystals of EOM 22A may receive the optical signal from light-emitting module 12 while EOM 22A is also receiving an electrical signal from feedback unit 26A of circuit 14. As such, the electrical signal may affect the electric field proximate to the crystals of EOM 22A, thus causing EOM 22A to modulate the optical signal. In some examples, EOM 22A modulates the optical signal by modulating the refractive index of the crystals using the electrical signal. EOM 22A, in some cases, may transmit the modulated optical signal to proof mass assembly 16. In some examples, EOM 22B is substantially similar to EOM 22A, with EOM 22B controlled by an electrical signal from feedback unit 26B.

Photoreceivers 24 may each include one or more transistors configured to absorb photons of an optical signal and output, in response to absorbing the photons, an electrical signal. In this manner, photoreceivers 24 may be configured to convert optical signals into electrical signals. Photoreceivers 24A, for example, may include a p-n junction that converts the photons of the optical signal into the electrical signal, where the electrical signal preserves at least some parameters of the optical signal. One or more frequency values and intensity values associated with the optical signal may be reflected in the electrical signal produced by photoreceivers 24A in response to photoreceivers 24A receiving the optical signal. For example, photoreceivers 24A may produce a stronger electrical signal (e.g., greater current magnitude) in response to receiving a stronger (e.g., greater power) optical signal. Additionally, in some cases, photoreceivers 24A may produce the electrical signal to reflect the one or more frequency values corresponding to the received optical signal. In other words, processing circuitry (e.g., processing circuitry 30) may analyze the electrical signal to determine the one or more frequency values corresponding to the optical signal. Photoreceivers 24A may include semiconductor materials such as any combination of Indium Gallium Arsenide, Silicon, Silicon Carbide, Silicon Nitride, Gallium Nitride, Germanium, or Lead Sulphide. In some examples, photoreceivers 24B is substantially similar to photoreceivers 24A.

Feedback units 26 may each include a set of circuit components for processing electrical signals. In some examples, the set of circuit components included in feedback unit 26A may include any combination of a band pass filter, a phase shifter, an electronic amplifier, and a voltage limiter. Such components may process, or filter, the electrical signal such that certain aspects of the electrical signal may be more efficiently measured (e.g., frequency values or intensity values). In the example of FIG. 1, feedback unit 26A may receive an electrical signal from photoreceiver 24A and output a processed electrical signal to EOM 22A, frequency counter 28A, and light-emitting module 12A. In this way, feedback unit 26A acts as a part of a first positive feedback loop by processing an electrical signal which EOM 22A uses to modulate an optical signal emitted by light-emitting module 12A, where the modulated optical signal passes through proof mass assembly 16 before arriving back at circuit 14 for processing by feedback unit 26A. The first positive loop may be configured to provide a tuning signal for driving light-emitting module 12A to a central frequency, which may help to reject noise due that causes fluctuations in frequency and/or amplitude in the optical signal output by light-emitting module 12A.

Feedback unit 26B may be substantially similar to feedback unit 26A in that feedback unit 26B receives an electrical signal from photoreceiver 24B and delivers a processed electrical signal to frequency counter 28B, EOM 22B, and light-emitting module 12B. As such, feedback unit 26B operates within a second feedback loop in a similar manner to which feedback unit 26A operates within the first feedback loop. Again, feedback unit 26B may be omitted.

Frequency counters 28 are circuit components that are each configured for measuring a frequency of an electrical signal. For example, frequency counter 28A may determine one or more frequency values corresponding to the processed electrical signal produced by feedback unit 26A. Frequency counter 28A may measure frequency values corresponding to the processed electrical signal in real-time or near real-time, such that frequency counter 28A tracks the frequency values as a function of time. Frequency counter 28B may be substantially similar to frequency counter 28A, except frequency counter 28B receives an electrical signal from feedback unit 26B rather than from feedback unit 26A.

Processing circuitry 30, and circuit 14 generally, may include one or more processors that are configured to implement functionality and/or process instructions for execution within system 10. For example, processing circuitry 30 may be capable of processing instructions stored in a storage device (not illustrated in FIG. 1). Processing circuitry 30 may include, for example, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or equivalent discrete or integrated logic circuitry, or a combination of any of the foregoing devices or circuitry. Accordingly, processing circuitry 30 may include any suitable structure, whether in hardware, software, firmware, or any combination thereof, to perform the functions ascribed herein to processing circuitry 30. Processing circuitry 30, and circuit 14 may include only analog circuitry, only digital circuitry, or a combination of analog circuitry and digital circuitry. The term “processor” or “processing circuitry” may generally refer to any of the foregoing analog circuitry and/or digital circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry.

Proof mass assembly 16 may include a proof mass, a frame, a set of tethers, and a set of DETF structures. The proof mass, in some examples, is suspended within the frame by the set of tethers and the set of DETF structures. For example, proof mass assembly 16 may include a set of DETF structures that suspend the proof mass in a first direction relative to the frame. Additionally, the set of tethers may suspend the proof mass in a second direction and a third direction relative to the frame. The first direction, the second direction, and the third direction may represent three axes (e.g., x-axis, y-axis, and z-axis) of a Cartesian space. In some cases, the set of DETF structures enable the proof mass to be displaced in the first direction. Additionally, in some cases, the set of tethers prevent the proof mass from being displaced in the second direction and the third direction. In this way, proof mass assembly 16 may only allow the proof mass to be displaced along a single axis (e.g., a displacement axis). Because the displacement of the proof mass may determine the acceleration measured by circuit 14, system 10 may be configured to determine the acceleration relative to the displacement axis.

In some examples, the first positive feedback loop (e.g., light-emitting module 12A, proof mass assembly 16, EOM 22A, photoreceiver 24A, feedback unit 26A, and frequency counter 28A) and the second positive feedback loop (e.g., light-emitting module 12B, proof mass assembly 16, EOM 22B, photoreceiver 24B, feedback unit 26B, and frequency counter 28B) are configured to independently determine an acceleration value representative of an acceleration of an object including system 10. For example, light-emitting module 12 may emit an optical signal, EOM 22A may modulate the optical signal to obtain a first modulated optical signal, and EOM 22A may transmit the first modulated optical signal to proof mass assembly 16. Photoreceiver 24A may receive the first modulated optical signal from proof mass assembly 16, where properties of the first modulated optical signal received by photoreceiver 24A may be affected by mechanical vibrations of a first DETF structure of proof mass assembly 16. Photoreceiver 24A converts the first modulated optical signal into a first electrical signal and transmits the first electrical signal to feedback unit 26A.

Feedback unit 26A may process the first electrical signal to obtain a first processed electrical signal. For example, feedback unit 26A may use any combination of a first band pass filter, a first phase shifter, a first electronic amplifier, and a first voltage limiter to process the first electrical signal. Frequency counter 28A may receive the first processed electrical signal and determine a first frequency value corresponding to the first processed electrical signal. In some cases, the first frequency value represents a mechanical vibration frequency of the first DETF structure of proof mass assembly 16, which carries the first modulated optical signal ultimately received by photoreceiver 24A.

In addition to transmitting the first processed electrical signal to frequency counter 28A, feedback unit 26A may transmit the first processed electrical signal to EOM 22A. In turn, EOM 22A may modulate the optical signal emitted by light-emitting module 12 based on the first processed electrical signal, where the first modulated optical signal is transmitted to photoreceiver 24A via the first DETF structure of proof mass assembly 16, thus completing the first positive feedback loop. As such, a future mechanical vibration frequency of the first DETF structure depends, at least in part, on a current mechanical vibration frequency of the first DETF structure.

Additionally, in some examples, the second positive feedback loop may determine a second frequency value. For example, light-emitting module 12 may emit an optical signal, EOM 22B may modulate the optical signal to obtain a second modulated optical signal, and EOM 22B may transmit the second modulated optical signal to proof mass assembly 16. Photoreceiver 24B may receive the second modulated optical signal from proof mass assembly 16, where properties of the second modulated optical signal received by photoreceiver 24B may be affected by mechanical vibrations of a second DETF structure of proof mass assembly 16. Photoreceiver 24B converts the second modulated optical signal into a second electrical signal and transmits the second electrical signal to feedback unit 26B.

In some examples, feedback unit 26B processes the second electrical signal to obtain a second processed electrical signal. For example, feedback unit 26B may use any combination of a second band pass filter, a second phase shifter, a second electronic amplifier, and a second voltage limiter to process the second electrical signal. Frequency counter 28B may receive the second processed electrical signal and determine a second frequency value corresponding to the second processed electrical signal. In some cases, the second frequency value represents a mechanical vibration frequency of the second DETF structure of proof mass assembly 16, which carries the second modulated optical signal ultimately received by photoreceiver 24B.

In addition to transmitting the second processed electrical signal to frequency counter 28B, feedback unit 26B may transmit the second processed electrical signal to EOM 22B. In turn, EOM 22B may modulate the optical signal emitted by light-emitting module 12 based on the second processed electrical signal, where the second modulated optical signal is transmitted to photoreceiver 24B via the second DETF structure of proof mass assembly 16, thus completing the second positive feedback loop. As such, a future mechanical vibration frequency of the second DETF structure depends, at least in part, on a current mechanical vibration frequency of the second DETF structure.

Processing circuitry 30 may be configured to calculate, based on the first frequency value, a first acceleration value. In some examples, to calculate the first acceleration value, processing circuitry 30 may subtract a baseline frequency value from the first frequency value to obtain a first frequency difference value. The baseline frequency value may represent a resonant mechanical frequency of the first DETF structure of proof mass assembly 16 while the proof mass is not displaced from a resting point along the proof mass displacement axis. In other words, the modulated optical signal emitted by EOM 22A may induce the first DETF structure to vibrate at the baseline frequency value while the proof mass is not displaced from the resting point along the proof mass displacement axis. As such, when the object is not accelerating, the first frequency difference value may be equal to zero since the first acceleration value—which represents the mechanical frequency of the first DETF structure—is equal to the baseline frequency value when the proof mass is not displaced (e.g., the object carrying system 10 is not accelerating). The first frequency difference value, in some examples, may be correlated with an acceleration of the object. In other words, an increase of a magnitude of the first frequency difference value may indicate an increase in the acceleration of the object and a decrease of a magnitude of the first frequency difference value may indicate decrease in the acceleration of the object.

Additionally, processing circuitry 30 may be configured to calculate a second acceleration value based on the second acceleration value. In some examples, to calculate the second acceleration value, processing circuitry 30 may subtract a baseline frequency value from the second frequency value to obtain a second frequency difference value. The second frequency difference value, in some examples, may be correlated with an acceleration of the object. In other words, an increase of a magnitude of the second frequency difference value may indicate an increase in the acceleration of the object and a decrease of a magnitude of the second frequency difference value may indicate decrease in the acceleration of the object. The first acceleration value and the second acceleration value, which are calculated by processing circuitry 30, may, in some cases, be approximately equal.

FIG. 2 is a block diagram illustrating circuit 14 of FIG. 1 in further detail, in accordance with one or more techniques of this disclosure. As illustrated in FIG. 1, circuit 14 includes EOMs 22, photoreceivers 24, feedback units 26, frequency counters 28, and processing circuitry 30. Feedback units 26 may each include band pass filters 40A. 40B (collectively, “band pass filters 40”), phase shifters 42A, 42B (collectively, “phase shifters 42”), electronic amplifiers 44A, 44B (collectively, “electronic amplifiers 44), noise rejection modules 45A, 45B (collectively, “noise rejection modules 45), and drivers 47A. 47B (collectively, “drivers 47). The first feedback loop includes band pass filter 40A, phase shifter 42A, electronic amplifier 44A, noise rejection module 45A, and driver 47A). The second feedback loop includes band pass filter 40B, phase shifter 42B, electronic amplifier 44B, noise rejection module 45B, and driver 47B.

Circuit 14 may be configured to configured to receive modulated optical signals from proof mass assembly 16, convert the optical signals into electrical signals, process the electrical signals, analyze the processed electrical signals to determine acceleration values, and use the processed electrical signals to modulate optical signals and reject noise, thus completing the first feedback loop and the second feedback loop. For example, photoreceivers 24A may receive a first modulated optical signal from a first DETF structure of proof mass assembly 16. The first modulated optical signal may include a frequency component associated with the first DETF structure itself, such as a vibration frequency of the first DETF structure. Photoreceivers 24A may convert the first modulated optical signal into a first set of electrical signals, preserving the frequency component indicative of the vibration frequency of the first DETF structure for noise rejection module 45A and driver 47A. Photoreceivers 24A may transmit the first set of electrical signals to feedback unit 26A, which includes band pass filter 40A, phase shifter 42A, electronic amplifier 44A, noise rejection module 45A, and driver 47A.

Band pass filter 40A may be an electronic filter that attenuates frequencies outside of a frequency range and “passes” frequencies within the frequency range. In some examples, band pass filter 40A includes any combination of passive filters, active filters, infinite impulse response (IIR) filters, finite impulse response (FIR) filters, Butterworth filters, Chebyshev filters, elliptic filters, Bessel filters, Gaussian filters, Legendre filters, or Linkwitz-Riley filters. In some examples, band pass filter 40A includes a combination of a high pass filter which passes frequencies above a high pass cutoff point and a low pass filter which passes frequencies below a low pass cutoff point. In some cases, band pass filter 40A passes frequencies within a range between 100 kilohertz (kHz) and 10,000 kHz.

Phase shifter 42A may be configured to shift a phase of the first electrical signal and the second electrical signal. Phase may be characterized as a position of an instant on a waveform cycle of a periodic waveform. For example, the first electrical signal may include periodic waveforms which represent frequency components of the first electrical signal. A maximum peak of a sine wave for example, may be at a different phase than a minimum peak, or a zero crossing of the sine wave. In some examples, phase shifter 42A may “delay” the first electrical signal by a time value in order to shift a timeline in which frequency components of the first electrical signal oscillate and delay the second electrical signal by a time value in order to shift a timeline in which frequency components of the second electrical signal oscillate.

Electronic amplifier 44A may amplify the first electrical signal and/or the second electrical signal such that an amplitude of the first electrical signal is increased by a gain factor. In other words, electronic amplifier 44A may increase a power of the first electrical signal and second electrical signal. By amplifying the first electrical signal and second electrical signal using electronic amplifier 44A, circuit 14 may improve an ability of processing circuitry 30 to analyze the first electrical signal and the second electrical signal, and modulate the optical signal emitted by light-emitting module 12 using EOM 22A.

Electronic amplifier 44A may include, in some cases, power amplifiers, operational amplifiers, or transistor amplifiers, or any combination thereof. Additionally, in some examples, electronic amplifier 44A is configured to limit a voltage of the first electrical signal and/or second electrical signal to a maximum voltage value. In other words, electronic amplifier 44A may prevent the first electrical signal and the second electrical signal from exceeding the maximum voltage value, meaning that the first processed electrical signal and the second processed electrical signal produced by feedback unit 26A may not exceed the maximum voltage value.

In some examples, the first set of electrical signals may pass through feedback unit 26A in an order from band pass filter 40A, to phase shifter 42A, to electronic amplifier 44A, and to noise rejection module 45A and driver 47A, as illustrated in FIG. 1. However, the order illustrated in FIG. 1 is not limiting. Band pass filter 40A, phase shifter 42A, and electronic amplifier 44A may be arranged to process the first electrical signal and second first electrical signal in any order.

Noise rejection module 45A may be configured to mitigate the effects of fluctuations in amplitude and/or frequency. For example, noise rejection module 45A may be configured to generate a tuning signal using a difference between a DC intensity level of a first optical frequency component in the first modulated optical signal corresponding to an optical resonance of proof mass assembly 16 plus one quarter of the Full Width at Half Maximum (FWHM) of the optical resonance and a DC intensity level of a second optical frequency component in the first modulated optical signal corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance. Because both optical frequencies are created in an intrinsically symmetric manner, noise rejection module 45A may be configured to preserve the equal intensity at each optical frequency of the first optical frequency component and the second optical frequency component such that light-emitting module 12A outputs an optical signal driven to an optical resonance of proof mass assembly 16 while rejecting frequency and/or amplitude fluctuations arising from, for example, photoreceiver aging.

Driver 47A may be configured to cause EOM 22A to drive the optical signal to a mechanical resonance of proof mass assembly 16. For example, driver 47A may be configured to generate a mechanical resonance feedback signal that causes EOM 22A to operate near or at the mechanical resonance of proof mass assembly 16. For example, driver 47A may generate the mechanical resonance feedback signal using a signal generator set to the mechanical resonance of proof mass assembly 16.

Driver 47A may transmit a mechanical resonance feedback signal to frequency counter 28A. Frequency counter 28A may determine a first frequency value, and processing circuitry 30 may determine a first acceleration value based on the first frequency value. Additionally, driver 47A may transmit the mechanical resonance feedback signal to EOM 22A and EOM 22A may modulate the optical signal emitted by light-emitting module 12A based on the mechanical resonance feedback signal. In this way, proof mass assembly 16, photoreceiver 24A, band pass filter 40A, phase shifter 42A, electronic amplifier 44A, noise rejection module 45A, driver 47A, EOM 22A, and frequency counter 28A are a part of the first positive feedback loop which produces the first acceleration value associated with the object including system 10.

The components of feedback unit 26B (e.g., band pass filter 40B, phase shifter 42BN, electronic amplifier 44B, noise rejection module 45B, and driver 47B) may be substantially similar to the respective components of feedback unit 26A. As such, the second positive feedback loop may be substantially similar to the first positive feedback loop.

FIG. 3 illustrates a conceptual diagram of proof mass assembly 16 including a proof mass 50 suspended within a frame 52 by a first DETF structure 54, a second DETF structure 58, and a set of tethers 62A-62R, in accordance with one or more techniques of this disclosure. As illustrated in FIG. 3, proof mass assembly 16 includes proof mass 50, frame 52, first DETF structure 54 including a first pair of mechanical beams 56A, 56B (collectively, “first pair of mechanical beams 56”), second DETF structure 58 including a second pair of mechanical beams 60A, 60B (collectively, “second pair of mechanical beams 60”), tethers 62A-62R (collectively, “tethers 62”), first distal tine 64, and second distal tine 68. Proof mass assembly 16 is aligned relative to proof mass displacement axis 72 and proof mass resting plane 74, as illustrated in FIG. 3.

Proof mass assembly 16 is a mechanical component of electro-opto-mechanical system 10. Because system 10 measures acceleration, which is a rate in which a velocity of an object changes over time, it may be beneficial to include proof mass assembly 16 so that acceleration can be measured based on a physical object such as proof mass 50. For example, system 10, which includes proof mass assembly 16 may be fixed to or included within an object. Consequently, as the object accelerates at an acceleration value, proof mass assembly 16 may also accelerate at the acceleration value. Acceleration may affect a position of proof mass 50 within frame 52 relative to proof mass displacement axis 72 and proof mass resting plane 74. For example, non-zero acceleration may cause proof mass 50 to be displaced from proof mass resting plane 74 along proof mass displacement axis 72. As described herein, if proof mass 50 is “displaced,” a center of mass of proof mass 50 is displaced relative to frame 52. Increasing a magnitude of acceleration may cause the displacement of proof mass 50 along proof mass displacement axis 72 to increase. Additionally, decreasing a magnitude of acceleration may cause the displacement of proof mass 50 along proof mass displacement axis 72 to decrease.

In some examples, proof mass 50 take the form of a patterned thin film, where the thin film has a mass within a range between 100 nanograms (ng) and 10.000 ng. Additionally, in some cases, the thin film has a thickness within a range between 1 nm and 5,000 nm. Proof mass 50 may be suspended within frame 52 along proof mass displacement axis 72 by first DETF structure 54 and second DETF structure 58 (collectively, “DETF structures 54, 58”). First DETF structure 54 and second DETF structure 58 may each have a high level of stiffness. For example, a scale factor of each of first DETF structure 54 and second DETF structure 58 may be within a range between 0.1 parts per million per gram (ppm/g) and 10 ppm/g. In this way, proof mass assembly 16 may include a very light proof mass 50 which is secured by very stiff DTEF structures 54, 58. As such, a very high acceleration (e.g., 100,000 m/s²) may cause proof mass 50 to be displaced along the proof mass displacement axis 72 by a very small displacement value, for example. In some examples, proof mass 50 is displaced along the proof mass displacement axis 72 by a displacement value of up to 100 nm.

To generate acceleration values indicative of the acceleration of the object in which system 10 is fixed to, system 10 may quantify, using optical signals, the displacement of proof mass 50 within frame 52. To quantify the displacement of proof mass 50, system 10 may measure and analyze mechanical properties of DETF structures 54, 58, such as mechanical vibrating frequency values corresponding to DETF structures 54, 58. Indeed, since DETF structures 54, 58 suspend proof mass 50, the mechanical vibrating frequencies of DETF structures 54, 58 may be affected due to a displacement of proof mass 50. For example, a displacement of proof mass 50 towards first DETF structure 54 and away from second DETF structure 58 may cause proof mass 50 to apply a compression force to first DETF structure 54 and apply a tension force to second DETF structure 58. Such a compression force may cause the mechanical vibration frequency of first DETF structure 54 to decrease and such a tension force may cause the mechanical vibration force of second DETF structure 58 to increase. Changes in the mechanical vibration frequencies of DETF structures 54, 58 may, in some examples, be proportional to the displacement of proof mass 50 relative to frame 52 in the direction of proof mass displacement axis 72. In some examples, System 10 may measure changes in the mechanical vibration frequencies of DETF structures 54, 58 by transmitting modulated optical signals through DETF structures 54, 58.

First DETF structure 54 may include, for example, the first pair of mechanical beams 56 separated by a gap. The first pair of mechanical beams 56 may include photonic crystal mechanical beams that are configured for carrying a first modulated optical signal while first DETF structure 54 is oscillating at a first mechanical vibrating frequency. In some cases, the first modulated optical signal is emitted by light-emitting module 12 (illustrated in FIG. 1), and the first modulated optical signal itself induces vibration in first DETF structure 54. Additionally, the vibration of the first DETF structure 54 may affect certain properties of the first modulated optical signal such that the mechanical vibrating frequency of the first DETF structure 54 is reflected in the first modulated optical signal. In this way, the first modulated optical signal may cause the mechanical vibration in the first DETF structure 54 and enable system 10 to measure the mechanical vibration frequency of the first DETF structure 54 based on the first modulated optical signal.

Additionally, second DETF structure 58 may include, for example, the second pair of mechanical beams 60 separated by a gap. The second pair of mechanical beams 60 may include photonic crystal mechanical beams that are configured for carrying a second modulated optical signal while second DETF structure 58 is oscillating at a second mechanical vibrating frequency. In some cases, the second modulated optical signal is emitted by light-emitting module 12 (illustrated in FIG. 1), and the second modulated optical signal itself induces vibration in second DETF structure 58. Additionally, the vibration of the second DETF structure 58 may affect certain properties of the second modulated optical signal such that the mechanical vibrating frequency of the second DETF structure 58 is reflected in the second modulated optical signal. In this way, the second modulated optical signal may cause the mechanical vibration to occur in the second DETF structure 58 and enable system 10 to measure the mechanical vibration frequency of the second DETF structure 58 based on the second modulated optical signal.

Proof mass 50 may be fixed to frame 52 by tethers 62. In some examples, tethers 62 may suspend proof mass 50 in proof mass resting plane 74 such that the center of mass of proof mass 50 does not move within proof mass resting plane 74 relative to frame 52. Proof mass displacement axis 72 may represent a single axis (e.g., x-axis) of a Cartesian space, and proof mass resting plane 74 may represent two axes (e.g., y-axis and z-axis) of the Cartesian space. Since tethers 62 may restrict proof mass 50 from being displaced relative to proof mass resting plane 74, in some examples, proof mass 50 may only be displaced along the proof mass displacement axis 72. System 10 may measure an acceleration based on mechanical vibrating frequencies of DETF structures 54, 58, where the mechanical vibrating frequencies are related to an amount of displacement of proof mass 50 along proof mass displacement axis 72. In this way, the acceleration determined by system 10 may be an acceleration relative to proof mass displacement axis 72.

First DETF structure 54 may include a proximal end that is proximate to proof mass 50, and a distal end that is separated from frame 52 by a first gap 66. First distal tine 64 may help to suspend first DETF structure 54 within frame 52 such that the first DETF structure 54 is perpendicular to proof mass resting plane 74. In some examples, first distal tine 64 extends perpendicularly to proof mass displacement axis 72 between two sidewalls of frame 52. An optical signal may travel through frame 52 via a first optical fiber (not illustrated in FIG. 3), the optical signal being coupled across first gap 66 to first DETF structure 54.

Second DETF structure 58 may include a proximal end that is proximate to proof mass 50, and a distal end that is separated from frame 52 by a second gap 70. Second distal tine 68 may help to suspend first DETF structure 58 within frame 52 such that the second DETF structure 58 is perpendicular to proof mass resting plane 74. In some examples, second distal tine 68 extends perpendicularly to proof mass displacement axis 72 between two sidewalls of frame 52. An optical signal may travel through frame 52 via a second optical fiber (not illustrated in FIG. 3), the optical signal being coupled across second gap 70 to second DETF structure 58.

FIG. 4 illustrates a conceptual diagram of system 10, in accordance with one or more techniques of this disclosure. The conceptual diagram of FIG. 4 includes light-emitting modules 12, components of circuit 14, and proof mass assembly 16.

In some examples, an object may be fixed to system 10. The object, in some cases, may accelerate. System 10, including proof mass assembly 16, may accelerate with the object. As proof mass assembly 16 accelerates, proof mass 50 may be displaced relative to frame 52. In the example illustrated in FIG. 4, if proof mass assembly 16 accelerates in direction 78, proof mass 50 is displaced in direction 78. Direction 78, in some examples, is aligned with a proof mass displacement axis (e.g., proof mass displacement axis 72 of FIG. 3. As proof mass 50 is displaced in direction 78 relative to frame 52, proof mass 50 applies a compression force to first DETF structure 54, and proof mass 50 applies a tension force to second DETF structure 58. Such forces may affect mechanical vibrating frequencies of DETF structures 54, 58, where mechanical vibration is induced in first DETF structure 54 and second DETF structure 58 by electro-optic modulator 22A and electro-optic modulator 22B, respectively. For example, the compression force applied to first DETF structure 54 may cause the mechanical vibration frequency of first DETF structure 54 to decrease, and the tension force applied to second DETF structure 58 may cause the mechanical vibration frequency of second DETF structure 58 to increase.

Light-emitting modules 12 may emit optical signals to EOMs 22. For example, light-emitting module 12A may emit a first optical signal comprising a first optical frequency component corresponding to an optical resonance of first DETF structure 54 plus one quarter of the FWHM of the optical resonance, and a second optical frequency component corresponding to the optical resonance of first DETF structure 54 minus one quarter of the FWHM of the optical resonance. In some examples, light-emitting module 12B may emit a first optical signal comprising a first optical frequency component corresponding to an optical resonance of second DETF structure 58 plus one quarter of the FWHM of the optical resonance, and a second optical frequency component corresponding to the optical resonance of second DETF structure 58 minus one quarter of the FWHM of the optical resonance.

In turn, EOM 22A and EOM 22B may modulate the optical signal according to a first processed electrical signal produced by feedback unit 26A and a second processed electrical signal produced by feedback unit 26B, respectively. As such, EOM 22A may produce a first modulated optical signal and EOM 22B may produce a second modulated optical signal. EOM 22A, for example, may transmit the first modulated optical signal to proof mass assembly 16. The first modulated optical signal may cross frame 52. In some examples, frame 52 includes an aperture or another opening bridged by a first optical fiber which allows the first modulated optical signal to pass. Additionally, the first modulated optical signal may couple across first gap 66 to the first DETF structure 54.

The first modulated optical signal may propagate through first DETF structure 54, inducing mechanical vibration in first DETF structure 54. In some examples, the first modulated optical signal propagates the length of first DETF structure 54 towards proof mass 50 along mechanical beam 56A and subsequently propagates the length of first DETF structure 54 away from proof mass 50 along mechanical beam 56B. In some examples, the first modulated optical signal propagates the length of first DETF structure 54 towards proof mass 50 along mechanical beam 56B and subsequently propagates the length of first DETF structure 54 away from proof mass 50 along mechanical beam 56A. In any case, by propagating the length of first DETF structure 54, the first modulated optical signal may retain information indicative of mechanical properties (e.g., the mechanical vibration frequency) of first DETF structure 54. After the first modulated optical signal propagates through first DETF structure 54, the first modulated optical signal may exit proof mass assembly 16 via first gap 66 and the first optical fiber of frame 52.

After exiting proof mass assembly 16, the first modulated optical signal, which may include fluctuations in amplitude and/or frequency, may arrive at photoreceiver 24A. Photoreceivers 24A convert the first modulated optical signal into a set of electrical signals for rejecting noise in light-emitting module 12A and for driving EOM 22A to a mechanical resonance of proof mass assembly 16. Frequency counter 28A may determine a first frequency value corresponding to the first processed electrical signal, where the first frequency value is indicative of the mechanical vibrating frequency of the first DETF structure 54. Processing circuitry 30 may subtract a baseline frequency value from the first frequency value to obtain a first frequency difference value and calculate a first acceleration value based on the first frequency difference value. EOM 22A may use the first processed electrical signal to modulate the optical signal emitted by light-emitting module 12.

Feedback unit 26A (e.g., noise rejection module 45A) may be configured to drive light-emitting module 12A to mitigate the effects of fluctuations in amplitude and/or frequency. For example, feedback unit 26A may be configured to generate a tuning signal using a difference between a DC intensity level of a first optical frequency component and a DC intensity level of a second optical frequency. In this way, feedback unit 26A may help to drive light-emitting module 12A to output an optical signal driven to an optical resonance of proof mass assembly 16 while rejecting frequency and/or amplitude fluctuations arising from, for example, photoreceiver aging.

EOM 22B, for example, may transmit the second modulated optical signal to proof mass assembly 16. The second modulated optical signal may cross frame 52. In some examples, frame 52 includes an aperture or another opening bridged by a second optical fiber which allows the second modulated optical signal to pass. Additionally, the second modulated optical signal may couple across second gap 70 to the second DETF structure 58. The second modulated optical signal may propagate through second DETF structure 58, inducing mechanical vibration in second DETF structure 58. In some examples, the second modulated optical signal propagates the length of second DETF structure 58 towards proof mass 50 along mechanical beam 60A and subsequently propagates the length of second DETF structure 58 away from proof mass 50 along mechanical beam 60B. In some examples, the second modulated optical signal propagates the length of second DETF structure 58 towards proof mass 50 along mechanical beam 60B and subsequently propagates the length of second DETF structure 58 away from proof mass 50 along mechanical beam 60A. In any case, by propagating the length of second DETF structure 58, the second modulated optical signal may retain information indicative of mechanical properties (e.g., the mechanical vibration frequency) of second DETF structure 58. After the second modulated optical signal propagates through second DETF structure 58, the second modulated optical signal may exit proof mass assembly 16 via second gap 70 and the second optical fiber of frame 52.

After exiting proof mass assembly 16, the second modulated optical signal, which may include thermal noise, may arrive at photoreceivers 24B. Photoreceivers 24B convert the second modulated optical signal into a set of electrical signals for rejecting noise in light-emitting module 12B and a second electrical signal for driving EOM 22B to a mechanical resonance of proof mass assembly 16. Frequency counter 28B may determine a second frequency value corresponding to the second processed electrical signal, where the second frequency value is indicative of the mechanical vibrating frequency of the second DETF structure 58. Processing circuitry 30 may subtract a baseline frequency value from the second frequency value to obtain a second frequency difference value and calculate a second acceleration value based on the second frequency difference value. EOM 22B may use the second processed electrical signal to modulate the optical signal emitted by light-emitting module 12.

Feedback unit 26B (e.g., noise rejection module 45B) may be configured to drive light-emitting module 12B to mitigate the effects of fluctuations in amplitude and/or frequency. For example, feedback unit 26B may be configured to generate a tuning signal using a difference between a DC intensity level of a first optical frequency component and a DC intensity level of a second optical frequency. In this way, feedback unit 26B may help to drive light-emitting module 12B to output an optical signal driven to an optical resonance of proof mass assembly 16 while rejecting frequency and/or amplitude fluctuations arising from, for example, photoreceiver aging.

FIG. 5 depicts additional aspects of system 10, in accordance with one or more techniques of this disclosure. For example, FIG. 5 illustrates the first DETF structure 54 including the first pair of mechanical beams 56. The optical signal emitted by light-emitting module 12 may induce a force between the first pair of mechanical beams 56, and the force may be modelled by a spring force. FIG. 5 illustrates a spring force provided by laser light between beams in an optical zipper in the gap between photonic crystal mechanical beams 56A, 56B of DETF structure 54 (502), a perspective view depiction of vibration modes in beams in an optical zipper in one common direction together (504), and a perspective view depiction of vibration modes in beams in an optical “zipper” in opposing directions of oscillation (506).

FIG. 6 is a conceptual diagram illustrating example techniques for noise rejection for an optomechanical device, in accordance with one or more techniques of this disclosure. As shown, electro-opto-mechanical system 610, which may be an example of system 10, may include light-emitting module 612, EOM 622, optical circulator 672, proof mass assembly 616 (also referred to herein as “zipper 616”), and feedback unit 626.

Light-emitting module 612 may include laser 611, signal generator 615, EOM 613, isolator 617, fiber bragg grating filter 619, and intensity stabilizer 680. Laser 611 produces a beam or optical signal at the optical resonance frequency of an optical mechanical device (e.g., a DETF structure of proof mass assembly 616 or another structure of proof mass assembly 616). For example, laser 611 may be configured to generate an optical signal comprising a central carrier corresponding to an optical resonance of proof mass assembly 616 using a tuning signal output by feedback unit 626.

EOM 613 modulates the optical signal at a frequency of one quarter the width of the optical resonance (Γ/4, where Γ is the FWHM of the optical resonance). For example, EOM 613 modulates the optical signal in the range of 10 GHz. For instance. EOM 613 may be configured to modulate the optical signal at a frequency corresponding to one quarter of a FWHM of the optical resonance of proof mass assembly 616 to generate a partially modulated optical signal comprising the central carrier, a first optical frequency component corresponding to the optical resonance plus one quarter of the FWHM of the optical resonance, and a second optical frequency component corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance.

The partially modulated optical signal passes through isolator 617, which may be configured to prevent transmission of optical signals from fiber bragg grating filter 619 to EOM 613 and allow a transmission of optical signals from EOM 613 to fiber bragg grating filter 619. In this way, isolator 617 may mitigate undesirable feedback.

The partially modulated optical signal passes through a narrow resonance notch filter, here a fiber bragg grating filter 619, which suppresses the central carrier. For example, fiber bragg grating filter 619 may be configured to filter the partially modulated optical signal to remove the central carrier from the partially modulated optical signal to generate a filtered optical signal. Although the example of FIG. 6 uses a fiber bragg grating filter, in other examples, other filters may be used to suppress the central carrier. As such, fiber bragg grating filter 619 outputs a partially modulated optical signal comprising two frequencies of light in the optical field, with equal intensities. For example, fiber bragg grating filter 619 outputs a partially modulated optical signal comprising a first optical frequency component corresponding to the optical resonance frequency plus one quarter of the FWHM of the optical resonance, and a second optical frequency component corresponding to the optical resonance frequency minus one quarter of the FWHM of the optical resonance without the central carrier.

Intensity stabilizer 680 may be configured to regulate an intensity of the filtered optical signal regulate an intensity of the filtered optical signal to a predetermined light intensity value. For example, the filtered optical signal passes through a Variable Optical Attenuator (VOA) 671, which may be configured to attenuate a portion of the optical signal. Tap 673 may be configured to output a first portion of the optical signal output from VOA 671 to photoreceiver 675 and a second portion of the optical signal output from VOA 671 to EOM 622. In this example, photoreceiver 675 may be configured to stabilize the overall light level of the filtered optical signal. Because both optical frequencies of the first optical frequency component and the second optical frequency component are created in an intrinsically symmetric manner, intensity servo 677 may be configured to preserve the equal intensity at each optical frequency of the first optical frequency component and the second optical frequency component. However, the process described herein is resistant to or immune to minor intensity variations, or to photoreceiver aging, because small intensity optical spring shifts may be common and canceled by the other tone.

The filtered optical signal passes through EOM 622, which is configured to drive the mechanical resonance of proof mass assembly 616. For instance, EOM 622 may be configured to modulate the filtered optical signal to generate a modulated optical signal driven to the mechanical resonance of proof mass assembly 616. EOM 622 may be driven in closed-loop operation (as shown), or in open loop “scanning” operation, and EOM 622 may provide modulation on each tone.

Optical circulator 672 may be configured to output the modulated optical signal to proof mass assembly 16 and receive the modulated optical signal reflected from proof mass assembly 16. For example, the modulated optical signal passes into port ‘1’ of optical circulator 672 and out of port ‘2’ of optical circulator 672, where the modulated optical signal interacts with proof mass assembly 616 (in this instance, a zipper cavity measured in reflection). In this way, the first optical frequency component and the second optical frequency component share a common path from creation to proof mass assembly 616, which helps to ensure that any fluctuations in the optical field induced by environmental perturbations to the optical paths will be common mode.

After reflection back into port ‘2’ of optical circulator 672, the modulated optical signal is output from port ‘3’ of optical circulator 672 to feedback unit 626. Feedback unit 626 may be configured to generate the tuning signal using a difference between a DC intensity level of the first optical frequency component in the modulated optical signal after the modulated optical signal is reflected from proof mass assembly 616 and a DC intensity level of the second optical frequency component in the modulated optical signal after the modulated optical signal is reflected from proof mass assembly 616. As shown, feedback unit 626 may include dichroic 684, photoreceivers 624A, 624B (collectively, “photoreceivers 624”), splitter 686, driver 647, data acquisition module 628, attenuator 688, noise rejection module 645, and frequency servo 694. Driver 647, which may be an example of drivers 47A, 47B, may include High-Pass Filter (HPF) 646, gain module 674, and phase module 676. Noise rejection module 645, which may be an example of noise rejection modules 45A, 45B, may include Low-Pass Filter (LPF) 690A, 690B (collectively, “LPFs 690”), and comparator 692.

Because the two optical frequencies of modulated optical signal are at substantially different optical frequencies, dichroic 684 (or another fiber bragg grating, or other wavelength selective optical splitting element) may separate the two optical frequencies of the modulated optical signal into a first portion of the modulated optical signal and a second portion of the modulated optical signal. For example, dichroic 684 may be configured to split, after the modulated optical signal is reflected from proof mass assembly 616, the modulated optical signal into a first portion of the modulated optical signal and a second portion of the modulated optical signal.

For example, dichroic 684 may be configured to split the modulated optical signal such that the first portion of the modulated optical signal comprises a set of frequency components that is different than a set of frequency components of the second portion of the modulated optical signal. For instance, dichroic 684 may be configured to split the modulated optical signal such that the first portion of the modulated optical signal comprising a set of frequency components that are less than (or greater than) a central carrier and the second portion of the modulated optical signal comprising a set of frequency components that are greater than (or less than) the central carrier. In some examples, dichroic 684 may be configured to split the modulated optical signal such that the first portion of the modulated optical signal comprises a first optical frequency component corresponding to the optical resonance plus (or minus) one quarter of the FWHM of the optical resonance and the second portion of the modulated optical signal comprises a second optical frequency component corresponding to the optical resonance minus (or plus) one quarter of the FWHM of the optical resonance.

Photoreceiver 624A may be configured to convert the first portion of the modulated optical signal into a first electrical signal. Similarly, photoreceiver 624B may be configured to convert the second portion of the modulated optical signal into a second electrical signal. Feedback unit 626 may be configured to generate a mechanical resonance feedback signal based on the first electrical signal and to generate a tuning signal based on the first electrical signal and the second electrical signal as follows.

Driver 647 may be configured to use high frequency beat notes formed by each separate optical frequency component with modulation sidebands of each separate optical frequency component (at the mechanical resonance frequency) to drive EOM 622 (e.g., in closed-loop operation), as well as to read out the acceleration. For example, feedback unit 626 may be configured to generate a mechanical resonance feedback signal based the first electrical signal received from photoreceiver 624A via splitter 686 and drive EOM 622 to modulate the filtered optical signal based on the mechanical resonance feedback signal. As shown, feedback unit 626 may include attenuator 688 to match an attenuation (e.g., −3 dB) resulting from splitter 686.

Data acquisition module 628 may be configured to measure, using the mechanical resonance feedback signal, an acceleration at proof mass assembly 616. Data acquisition module 628 may include or perform one or more functions of frequency counter 28A. For example, data acquisition module 628 may be configured to estimate a mechanical resonance frequency of proof mass assembly 616 using an electrical signal produced by photoreceiver 624 and filtered by HPF 646.

For example, HPF 646 may be configured to filter the first electrical signal to remove a DC intensity level intensity level from the first electrical signal. In this example, gain module 674 may be configured to apply, after filtering by HPF 646, a gain to the first electrical signal and phase shift module 676 may be configured to apply, after filtering by HPF 646, a phase shift to the first electrical signal after filtering by HPF 646 to generate a driving signal. In this example, EOM 622 may be configured to modulate the filtered optical signal using the driving signal.

Noise rejection module 645 may be configured to use DC levels of each separate optical frequency component to stabilize the frequency of laser 611, for instance, to keep laser 611 at the desired frequency relative to an optical resonance of proof mass assembly 616. The optical signal reflected back at each of the separate optical frequencies varies according to detuning of each at each of the separate optical frequencies from the optical resonance of proof mass assembly 616. For example, LPF 690A may be configured to filter the first electrical signal received from photoreceiver 624A via attenuator 688 to generate a first filtered electrical signal corresponding to the DC intensity level of the first optical frequency and LPF 690B may be configured to filter the second electrical signal received from photoreceiver 624B to generate a second filtered electrical signal corresponding to the DC intensity level of the second optical frequency. In this example, comparator 692 may be configured to compare the first filtered electrical signal and the second filtered electrical signal and generate the tuning signal. Frequency servo 694 may be configured to drive laser 611 to a central carrier using the tuning signal. In this way, feedback unit 626 may reject noise and hence improve a performance of the optomechanical device compared to systems that omit

FIG. 7 is a conceptual diagram illustrating an example optical signal 702, in accordance with one or more techniques of this disclosure. FIG. 7 is discussed with reference to FIGS. 1-6 for example purposes only. The abscissa axis (e.g., horizontal axis) of FIG. 7 represents wavelength in nanometers (nm) and the ordinate axis (e.g., vertical axis) of FIG. 7 represents a normalized intensity of an optical signal 702 output by laser 611. As shown, optical signal 702 comprises a central carrier 712, which may represent an optical resonance of proof mass assembly 616.

FIG. 8 is a conceptual diagram illustrating an example partially modulated optical signal 804, in accordance with one or more techniques of this disclosure. FIG. 8 is discussed with reference to FIGS. 1-7 for example purposes only. The abscissa axis (e.g., horizontal axis) of FIG. 8 represents wavelength in nanometers (nm) and the ordinate axis (e.g., vertical axis) of FIG. 8 represents a normalized intensity of a partially modulated optical signal 804 output by EOM 613. As shown, partially modulated optical signal 804 comprises a central carrier 812, which may represent an optical resonance of proof mass assembly 616, a first optical frequency component 816 corresponding to the optical resonance plus one quarter of the FWHM of the optical resonance, and a second optical frequency component 814 corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance.

FIG. 9 is a conceptual diagram illustrating an example filtered optical signal 906, in accordance with one or more techniques of this disclosure. FIG. 9 is discussed with reference to FIGS. 1-8 for example purposes only. The abscissa axis (e.g., horizontal axis) of FIG. 9 represents wavelength in nanometers (nm) and the ordinate axis (e.g., vertical axis) of FIG. 9 represents a normalized intensity of a filtered optical signal 906 output by fiber bragg grating filter 619. As shown, filtered optical signal 906 comprises a first optical frequency component 916 corresponding to the optical resonance plus one quarter of the FWHM of the optical resonance of proof mass assembly 616 and a second optical frequency component 914 corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance. That is, fiber bragg grating filter 619 removes the central carrier from partially modulated optical signal 804 to generate filtered optical signal 906.

FIG. 10 is a conceptual diagram illustrating an example modulated optical signal 1008, in accordance with one or more techniques of this disclosure. FIG. 10 is discussed with reference to FIGS. 1-9 for example purposes only. The abscissa axis (e.g., horizontal axis) of FIG. 10 represents wavelength in nanometers (nm) and the ordinate axis (e.g., vertical axis) of FIG. 10 represents a normalized intensity of a modulated optical signal 1008 output by EOM 622. As shown, modulated optical signal 1008 comprises a first optical frequency component 1016 corresponding to the optical resonance plus one quarter of the FWHM of the optical resonance of proof mass assembly 616 that is driven to a mechanical resonance of proof mass assembly 616 and a second optical frequency component 914 corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance that is driven to the mechanical resonance of proof mass assembly 616.

FIG. 11 is a conceptual diagram illustrating an example optical response 1109 of a resonance in reflection as a function of wavelength of the optical signal, in accordance with one or more techniques of this disclosure. FIG. 11 is discussed with reference to FIGS. 1-10 for example purposes only. The abscissa axis (e.g., horizontal axis) of FIG. 11 represents wavelength in nanometers (nm) and the ordinate axis (e.g., vertical axis) of FIG. 1 represents a normalized reflection of a modulated optical signal 1109 output by proof mass assembly 616. In the example of FIG. 11, the normalized reflection of a modulated optical signal 1109 resonates at a frequency 1110 corresponding to an optical resonance of proof mass assembly 616.

FIG. 12 is a conceptual diagram illustrating an example optical response of a first optical frequency component 1222 and a second optical frequency component 1220, in accordance with one or more techniques of this disclosure. FIG. 12 is discussed with reference to FIGS. 1-11 for example purposes only. The abscissa axis (e.g., horizontal axis) of FIG. 12 represents wavelength in nanometers (nm) and the ordinate axis (e.g., vertical axis) of FIG. 12 represents a normalized reflection 1209 of a modulated optical signal output by proof mass assembly 616. In the example of FIG. 12, the normalized reflection 1209 of a modulated optical signal resonates at a frequency 1210 corresponding to an optical resonance of proof mass assembly 616.

By measuring each DC level and comparing them, laser 611 may be tuned to lock the frequency of the laser to an optical resonance of proof mass assembly 616. For example, the two equal intensity tones of first optical frequency component 1222 and second optical frequency component 1220 cancel common mode amplitude fluctuations via the optical spring effect, which would otherwise write noise into the measured mechanical resonance of proof mass assembly 616. For instance, in response to first optical frequency component 1222 comprising a DC intensity level that is greater than a DC intensity level of second optical frequency component 1220, comparator 692 may generate a tuning signal to drive frequency servo 694 to cause laser 611 to decrease a frequency of a central carrier of the optical signal output by laser 611. In response, however, to first optical frequency component 1222 comprising a DC intensity level that is less than a DC intensity level of second optical frequency component 1220, comparator 692 may generate a tuning signal to drive frequency servo 694 to cause laser 611 to increase a frequency of a central carrier of the optical signal output by laser 611. In some examples, in response to first optical frequency component 1222 comprising a DC intensity level that corresponds to (e.g., is equal to) a DC intensity level of second optical frequency component 1220, comparator 692 may generate a tuning signal to drive frequency servo 694 to cause laser 611 to maintain a frequency of a central carrier of the optical signal output by laser 611. In this way, noise rejection module 645 may reduce the effects of laser amplitude noise while simultaneously providing a process for frequency stabilization to the optical resonance.

FIG. 13 is a flow diagram illustrating an example process for tracking a mechanical resonance of a proof mass assembly, in accordance with one or more techniques of this disclosure. FIG. 13 is described with respect to FIGS. 1-12 for example purposes only.

Laser 611 generates an optical signal comprising a central carrier corresponding to an optical resonance of proof mass assembly 616 using a tuning signal (1302). EOM 613 modulates the optical signal at a frequency corresponding to one quarter of a FWHM of the optical resonance of proof mass assembly 616 to generate a partially modulated optical signal comprising the central carrier, a first optical frequency component corresponding to the optical resonance plus one quarter of the FWHM of the optical resonance, and a second optical frequency component corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance (1304).

Fiber bragg grating filter 619 filters the partially modulated optical signal to remove the central carrier from the partially modulated optical signal to generate a filtered optical signal (1306). EOM 622 modulates the filtered optical signal to generate a modulated optical signal driven to the mechanical resonance of proof mass assembly 616 (1308). Optical circulator 672 outputs the modulated optical signal to the proof mass assembly and receives the modulated optical signal reflected from the proof mass assembly (1310).

Noise rejection module 645 generates the tuning signal using a difference between a DC intensity level of the first optical frequency component in the modulated optical signal and a DC intensity level of the second optical frequency component in the modulated optical signal (1312). HPF 646 generates, with splitter 686, photoreceiver 624A, and dichroic 684, a mechanical resonance feedback signal based on the modulated optical signal and data acquisition module 628 measures, using the mechanical resonance feedback signal, an acceleration at proof mass assembly 616 (1314). For example, HPF 646 generates the mechanical feedback signal by removing low frequency components (e.g., DC component) from an electrical signal output by photoreceiver 624A and data acquisition module 628 may measure one or more frequency values from the mechanical feedback signal.

The optomechanical device described herein may include only analog circuitry, only digital circuitry, or a combination of analog circuitry and digital circuitry. Digital circuitry may include, for example, a microcontroller on a single integrated circuit containing a processor core, memory, inputs, and outputs. For example, digital circuitry of the optomechanical device described herein may include one or more processors, including one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. The term “processor” or “processing circuitry” may generally refer to any of the foregoing analog circuitry and/or digital circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry.

Digital circuitry may utilize hardware, software, firmware, or any combination thereof for achieving the functions described. Those functions implemented in software may be stored on or transmitted over, as one or more instructions or code, a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer-readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure.

The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses that include integrated circuits (ICs) or sets of ICs (e.g., chip sets). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require realization by different hardware units. Rather, various units may be combined or provided by a collection of interoperative hardware units, including one or more processors as described above, in conjunction with suitable software and/or firmware. 

What is claimed is:
 1. An optomechanical device for tracking a mechanical resonance of a proof mass assembly, the optomechanical device comprising: the proof mass assembly; and a circuit configured to: generate an optical signal comprising a central carrier corresponding to an optical resonance of the proof mass assembly using a tuning signal; modulate the optical signal at a frequency corresponding to one quarter of a Full Width at Half Maximum (FWHM) of the optical resonance of the proof mass assembly to generate a partially modulated optical signal comprising the central carrier, a first optical frequency component corresponding to the optical resonance plus one quarter of the FWHM of the optical resonance, and a second optical frequency component corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance; filter the partially modulated optical signal to remove the central carrier from the partially modulated optical signal to generate a filtered optical signal; modulate the filtered optical signal to generate a modulated optical signal driven to the mechanical resonance of the proof mass assembly; and generate the tuning signal using a difference between a Direct Current (DC) intensity level of the first optical frequency component in the modulated optical signal and a DC intensity level of the second optical frequency component in the modulated optical signal.
 2. The optomechanical device of claim 1, wherein the circuit is configured to: output the modulated optical signal to the proof mass assembly and receive the modulated optical signal reflected from the proof mass assembly, wherein, to generate the tuning signal, the circuit is configured to generate the tuning signal based on the modulated optical signal after the modulated optical signal is reflected from the proof mass assembly.
 3. The optomechanical device of claim 2, wherein the circuit is configured to: split, after the modulated optical signal is reflected from the proof mass assembly, the modulated optical signal into a first portion of the modulated optical signal and a second portion of the modulated optical signal, wherein the first portion of the modulated optical signal comprises a set of frequency components that is different than a set of frequency components of the second portion of the modulated optical signal, and wherein, to generate the tuning signal, the circuit is configured to generate the tuning signal based on the first portion of the modulated optical signal and the second portion of the modulated optical signal.
 4. The optomechanical device of claim 3, wherein the first portion of the modulated optical signal comprises the first optical frequency component after the modulated optical signal is reflected from the proof mass assembly; or wherein the first portion of the modulated optical signal comprises the second optical frequency component after the modulated optical signal is reflected from the proof mass assembly.
 5. The optomechanical device of claim 3, wherein the circuit is configured to: convert the first portion of the modulated optical signal into a first electrical signal, and convert the second portion of the modulated optical signal into a second electrical signal, wherein, to generate the tuning signal, the circuit is configured to generate the tuning signal based on the first electrical signal and the second electrical signal.
 6. The optomechanical device of claim 5, wherein the circuit is configured to: filter the first electrical signal to generate a first filtered electrical signal corresponding to the DC intensity level of the first optical frequency; and filter the second electrical signal to generate a second filtered electrical signal corresponding to the DC intensity level of the second optical frequency, wherein, to generate the tuning signal, the circuit is configured to generate the tuning signal to drive based on a comparison of the first filtered electrical signal and the second filtered electrical signal.
 7. The optomechanical device of claim 5, wherein the circuit is configured to: generate a mechanical resonance feedback signal based the first electrical signal, wherein, to modulate the filtered optical signal, the circuit is configured to modulate the filtered optical signal based on the mechanical resonance feedback signal.
 8. The optomechanical device of claim 7, wherein the circuit is configured to: measure, using the mechanical resonance feedback signal, an acceleration at the proof mass assembly.
 9. The optomechanical device of claim 7, wherein, to generate the mechanical resonance feedback signal, the circuit is configured to: filter the first electrical signal to remove a DC intensity level intensity level from the first electrical signal.
 10. The optomechanical device of claim 9, wherein, to modulate the filtered optical signal, the circuit is configured to: apply a gain and phase shift to the first electrical signal after filtering to generate a driving signal; and modulate the filtered optical signal using the driving signal.
 11. The optomechanical device of claim 1, wherein the circuit is configured to: regulate an intensity of the filtered optical signal to a predetermined light intensity value before modulating the filtered optical signal.
 12. The optomechanical device of claim 1, wherein the proof mass assembly comprises a set of double-ended tuning fork (DETF) structures and wherein the mechanical resonance of the proof mass assembly is a mechanical resonance of the DETF structures.
 13. A method for tracking a mechanical resonance of a proof mass assembly, the method comprising: generating, by a circuit, an optical signal comprising a central carrier corresponding to an optical resonance of the proof mass assembly using a tuning signal; modulating, by the circuit, the optical signal at a frequency corresponding to one quarter of a Full Width at Half Maximum (FWHM) of the optical resonance of the proof mass assembly to generate a partially modulated optical signal comprising the central carrier, a first optical frequency component corresponding to the optical resonance plus one quarter of the FWHM of the optical resonance, and a second optical frequency component corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance; filtering, by the circuit, the partially modulated optical signal to remove the central carrier from the partially modulated optical signal to generate a filtered optical signal; modulating, by the circuit, the filtered optical signal to generate a modulated optical signal driven to the mechanical resonance of the proof mass assembly; and generating, by the circuit, the tuning signal using a difference between a Direct Current (DC) intensity level of the first optical frequency component in the modulated optical signal and a DC intensity level of the second optical frequency component in the modulated optical signal.
 14. The method of claim 13, comprising: outputting, by the circuit, the modulated optical signal to the proof mass assembly and receive the modulated optical signal reflected from the proof mass assembly, wherein generating the tuning signal comprises generating the tuning signal based on the modulated optical signal after the modulated optical signal is reflected from the proof mass assembly.
 15. The method of claim 14, comprising: splitting, by the circuit and after the modulated optical signal is reflected from the proof mass assembly, the modulated optical signal into a first portion of the modulated optical signal and a second portion of the modulated optical signal, wherein the first portion of the modulated optical signal comprises a set of frequency components that is different than a set of frequency components of the second portion of the modulated optical signal, and wherein generating the tuning signal comprises generating the tuning signal based on the first portion of the modulated optical signal and the second portion of the modulated optical signal.
 16. The method of claim 15, wherein the first portion of the modulated optical signal comprises the first optical frequency component after the modulated optical signal is reflected from the proof mass assembly; or wherein the first portion of the modulated optical signal comprises the second optical frequency component after the modulated optical signal is reflected from the proof mass assembly.
 17. The method of claim 15, comprising: converting, by the circuit, the first portion of the modulated optical signal into a first electrical signal; and converting, by the circuit, the second portion of the modulated optical signal into a second electrical signal, wherein generating the tuning signal comprises generating the tuning signal based on the first electrical signal and the second electrical signal.
 18. The method of claim 17, comprising: filtering, by the circuit, the first electrical signal to generate a first filtered electrical signal corresponding to the DC intensity level of the first optical frequency; and filtering, by the circuit, the second electrical signal to generate a second filtered electrical signal corresponding to the DC intensity level of the second optical frequency, wherein generating the tuning signal comprises generating the tuning signal to drive based on a comparison of the first filtered electrical signal and the second filtered electrical signal.
 19. The method of claim 17, comprising: generating, by the circuit, a mechanical resonance feedback signal based the first electrical signal, wherein modulating the filtered optical signal comprises modulating the filtered optical signal based on the mechanical resonance feedback signal.
 20. An optomechanical system for tracking a mechanical resonance of a proof mass assembly, the optomechanical system comprising: a circuit configured to: generate an optical signal comprising a central carrier corresponding to an optical resonance of the proof mass assembly using a tuning signal; modulate the optical signal at a frequency corresponding to one quarter of a Full Width at Half Maximum (FWHM) of the optical resonance of the proof mass assembly to generate a partially modulated optical signal comprising the central carrier, a first optical frequency component corresponding to the optical resonance plus one quarter of the FWHM of the optical resonance, and a second optical frequency component corresponding to the optical resonance minus one quarter of the FWHM of the optical resonance; filter the partially modulated optical signal to remove the central carrier from the partially modulated optical signal to generate a filtered optical signal; modulate the filtered optical signal to generate a modulated optical signal driven to the mechanical resonance of the proof mass assembly; and generate the tuning signal using a difference between a Direct Current (DC) intensity level of the first optical frequency component in the modulated optical signal and a DC intensity level of the second optical frequency component in the modulated optical signal. 